1. Field of the Invention
This invention relates generally to fixed-focus cameras and more particularly to a system and method for automatic engagement of a close-up lens in a fixed-focus camera.
2. Discussion of the Prior Art
Producing a well-focused image is a primary consideration of most camera users as well as camera manufacturers. Another important and sometimes conflicting goal is the cost of making a camera to produce such images. Fixed-focus cameras are relatively inexpensive due to their simple design, but are unable to focus on objects closer than a given focal length for the design of that fixed-focus lens. To circumvent this limitation, a retractable close-up lens with a shallow depth of field is often added to the lens assembly for photographing nearby subjects (typically one foot).
The addition of a close-up lens enables users to switch between the fixed-focus lens and the close-up lens, depending on the photographic subject's distance from the camera. The photographer can then determine which lens is likely to produce a focused image and switch to the appropriate lens. This task can sometimes be difficult as well as distracting, and must be performed for each and every picture taken to ensure consistent results. Many camera users cannot estimate distances accurately enough to effectively switch between using normal and close-up lenses. Also, the time necessary to estimate focal distance and switch to the appropriate lens may result in missed photographic opportunities. Even when a skilled user operates the camera, it is difficult to consistently produce well-focused images using a manual lens-switching system.
Previous attempts to overcome the lens-switching problem in fixed-focus cameras include active range finding schemes using infrared light or sonar to measure the distance from camera to photographic subject. The distance measurement can then be used to engage or disengage a close-up lens. Passive measurement schemes include providing lengths of cord or similar measuring devices whose lengths correspond to focal lengths of the respective lenses. The photographer may then use the measuring devices to determine proper lens configuration. These range finding schemes of the prior art add elaborate components to the camera and make the picture-taking process more complex and difficult.
Another prior art approach features an auto-focus camera with a focal-distance measuring system combined with a lens-adjusting mechanism. The adjusting mechanism automatically moves the lens through the entire range of lens focal lengths responsive to the focal distance measurement. Due to its increased complexity, the adjustable lens mechanism's cost is generally significantly higher than a comparable fixed-focus lens. Common auto focus systems are not applicable to fixed-focus cameras because a fixed-focus camera's lens is not movable or adjustable over a range of focal lengths.
Therefore, an improved system and method is needed to automatically engage and disengage a close-up lens in a fixed-focus camera, to economically permit capture of well-focused images with minimum difficulty and distraction to the camera user.